The present invention relates to elasticity imaging including but not limited to strain imaging and in particular to an improved method of determining displacement vectors used to produce such images.
Strain imaging produces images revealing the underlying elastic parameters of the material being measured. When used in medicine, strain imaging is analogous to palpation by a physician, that is, the pressing of tissue by the physician to feel differences in elasticity in the underlying structures.
In a common form of strain imaging, two separate images are obtained with the measured material in different states of deformation, typically, as deformed by a mechanical or physiological stimulus. In ultrasound strain imaging, the ultrasound probe itself may be used to provide this deformation.
The two images are analyzed to deduce the amount of displacement in the material at a number of corresponding regions. The gradient in these displacements, determined as a function of the spatial location of the regions, provides strain information generally reflecting the elasticity of the tissue. An example of such strain imaging and a description of techniques for determining displacement of tissue between two images are described in detail in U.S. Pat. No. 6,508,768 entitled: Ultrasonic Elasticity Imaging, and in pending U.S. application Ser. No. 12/258,532 filed Oct. 27, 2008 and entitled: Ultrasonic Strain Imaging Device with Selectable Cost-Function, and in pending U.S. application Ser. No. 12/645,936 filed Dec. 23, 2009 and entitled: Elasticity Imaging Device with Improved Characterization of Seed Displacement Calculations, all assigned to the same assignee as the present invention and hereby incorporated by reference.
The displacement between corresponding regions of the material in the first and second state of deformation can be determined by identifying a multi-point region (i.e. a reference kernel) in the material in the first state of deformation and moving this kernel within a two- or three-dimensional search region over a search region of the material in the second state of deformation. The displacement vector is determined by the best match between the reference kernel and its overlapping portion in the search region of the material in the second state of deformation (i.e. the target kernel). The best match may be determined by evaluating a similarity of the data of the reference and target kernels, for example, as a sum of the magnitude of differences between individual samples of these two kernels or other similar technique.
In ultrasonic imaging systems, the determination of displacement may be limited to an axial direction defined by the propagation of the ultrasonic signal. This is because motion tracking in the lateral direction (perpendicular to the axial direction) tends to be of low quality possibly because of the loss of phase information because sequential data in the lateral direction is assembled from multiple rather than a single beam.
Nevertheless lateral displacement information can be valuable because it provides a more complete picture of elasticity necessary for many types of measurement.